1. Field of the invention
This invention relates to electrochemical cells having a liquid alkali metal electrode. Such a cell makes use of a suitable cathodic reactant and has, between the alkali metal and cathodic reactant, a solid electrolyte in a form of a ceramic element, e.g. a membrane, that conducts ions of the alkali metal. A typical example of such a cell is a sodium-sulfur cell having a .beta.-alumina ceramic membrane forming the electrolyte.
2. Prior Art
When such a cell with a liquid alkali metal electrode and ceramic membrane is passing current, electrons flow from the alkali metal forming the anode to the cathodic reactant through the external circuit. Positive ions of the alkali metal pass through the ceramic and combine with the negative ions of the reactant. The effect of discharging the cell is therefore to cause the alkali metal to pass through the ceramic membrane constituting the electrolyte. The main limitation to the power that can be derived from such a cell is the resistance of the ceramic membrane and this is inversely proportional to the area in contact with the alkali metal and with the cathodic reactant. Thus, if during discharge of the cell, the alkali metal level falls as the alkali metal passes through the electrolyte, then the effective area of alkali metal in contact with the electrolyte decreases progressively causing a rapid increase in resistance. Apart from the loss of power so caused, the concentration of current flow through the decreasing area of ceramic may damage the ceramic membrane. For this reason therefore it has heretofore been considered necessary to include in the reservoir holding the alkali metal required for the reaction enough alkali metal to maintain the required level in contact with the electrolyte when the reaction is complete. This extra alkali metal, which does not contribute to the electrochemical process, is typically about one third of the total alkali metal.
Similarly, it is also necessary, for efficient operation, to keep the cathode surface of the electrolyte covered by the cathodic reactant. As the cell discharges and the alkali metal passes through the electrolyte, the volume of the cathodic reactant increases. The cathodic reactant would normally be within an annular region between the electrolyte surface and a current collector; it is desirable to keep the current path between the electrolyte and current collector as short as possible. However provision must be made to accommodate the increased volume of cathodic reactant as the cell discharges.
Quite apart from the utilisation of the total amount of alkali metal, it is not possible to increase the capacity of the cell in relation to the weight of alkali metal used, and power density by increasing the size of the effective electrode chamber in the cell. This may readily be seen by considering a cell having an electrode formed by a tube with the alkali metal inside the tube and the cathodic reactant outside the tube. If the power of the cell is increased by increasing the diameter of the ceramic tube, the surface area of the tube (and hence the power) increases linearly with increasing tube diameter but the weight of the unused alkali metal necessary to maintain the tube full would increase quadratically with increase of tube diameter.